Electrically Conductive Membrane-Assisted Anammox MBR via Extracellular Electron Transfer for Enhanced Nitrogen Removal under Nitrite-Deficient Conditions

Abstract

Conventional anaerobic ammonium oxidation (anammox) relies on nitrite (NO₂^–) as the electron acceptor for ammonium (NH₄⁺) oxidation. However, NO₂^– is often insufficient in ammonium-rich wastewater, necessitating complex pretreatment to meet the stoichiometric NH₄⁺: NO₂^– ratio of 1:1.32. Recent studies suggest that anammox bacteria (AnAOB) can perform extracellular electron transfer (EET), indicating a possible route to reduce NO₂^– demand. However, how to achieve efficient and stable EET under nitrite-deficient conditions remains unclear. Here, an electrochemical anammox membrane bioreactor equipped with conductive membranes (Amx-EMBR) was developed to couple AnAOB enrichment with EET-assisted nitrogen removal. By promoting the dense AnAOB accumulation on the anode, the efficiency of electron transfer from AnAOB to the anode was significantly enhanced, thereby enabling the system to realize EET-associated NO₂^– oxidation. With influent concentrations of 250 mg/L NH₄⁺ and 150 mg/L NO₂^–, the reactor achieved a total nitrogen removal efficiency of approximately 90%, corresponding to a 32.8% increase compared with the nonelectrified reactor. ¹⁵N-labeling, riboflavin, and hydroxylamine (NH₂OH) interference experiments confirmed the occurrence of an EET-driven nitrogen removal pathway. Meanwhile, metagenomic analysis further supported the genetic potential for electrode-dependent NH₄⁺ oxidation and extracellular electron transfer. Furthermore, the reactor exhibited reduced membrane fouling during long-term operation. This study demonstrates an effective strategy for enhancing nitrogen removal under nitrite-deficient conditions, breaking through the strict stoichiometric constraints of conventional anammox with respect to the influent ammonium-to-nitrite ratio.